Field of the Invention
The invention relates to a controllable oscillator circuit that can be tuned, for example, to set a transmitter or to compensate for tolerances over a defined frequency band.
A known LC oscillator includes a tuned circuit made up of an inductance having a capacitance disposed in parallel therewith as well as a resistance and a negative resistance for attenuation cancellation. The negative resistance -R usually is an amplifier with positive feedback in practice.
To tune the LC oscillator of this type, use is usually made of a variable capacitance diode disposed in place of the capacitance. The disadvantage of using a variable capacitance diode is, firstly, that it is difficult to integrate it into an integrated circuit, and, secondly, the limited frequency band that is achieved, which is usually 10 to 25% of the resonant frequency. In addition, normal P.sub.n junctions in integrated circuits usually have poor characteristics in terms of the maximum tuning voltage, the series resistance, the shape of the characteristic curve and the capacitance variation.